Properly dimension drinking water pipes.

the regulation is different for every water supplier.

The regulation comes from the Federal Government and was issued with the approval of the Bundesrat. In my opinion, it applies generally. Deviations are permissible under certain conditions; I guess that practically, especially regarding the point discussed here, there are no differing regulations.

because mounting a new faucet once

That could be understood as an insignificant change to a system. According to the regulation, only the installation and significant changes fall under the "installer obligation." I am not a lawyer ... layman's opinion, like all my statements here.

I recently changed my leaking shower hose by myself, anyway. I also connected my sink by myself (which is what you do when you get the kitchen from a carpenter and not from a turnkey kitchen builder).